In a laser processing apparatus that radiates a laser beam onto a workpiece and performs a laser processing on the workpiece, a processing lens converges the laser beam into a predetermined focus position so that the converged laser beam is radiated onto the workpiece. In such a laser processing apparatus, when the processing lens absorbs the laser beam, the refractive index of the processing lens changes. This phenomenon is called a thermal lens effect and could be a cause that changes the focus position of the laser beam. For this reason, such a laser processing apparatus is configured so as to correct deviations of the focus position caused by a thermal lens effect by adjusting the position of the processing lens in the optical axis direction, so that the laser beam is converged into a desired focus position.
For example, a laser processing apparatus described in Patent Document 1 is configured so that a test object is placed in a position (i.e., outside a processing table) that is different from an area in which a processing procedure is performed on a workpiece, for the purpose of detecting a deviation of the focus position. Before a laser processing is started, a laser beam is radiated onto the test object, so that the deviation of the focus position is measured based on a rise in the temperature of the test object. And also, based on a measurement result, the distance between the workpiece and the processing lens is corrected before the laser processing is started.
As another example, a laser processing apparatus according to Patent Document 2 is configured so that a processing nozzle is provided with a temperature detector for the purpose of detecting a deviation of the focus position. Based on a measurement result of the temperature detector, the distance between the workpiece and the processing lens for performing a laser processing is corrected.    Patent Document 1: Japanese Patent Application Laid-open No. S63-93491    Patent Document 2: Japanese Patent Application Laid-open No. S63-93492